11 research outputs found
Innovative methods for the characterization of ceramic nanofiltration membranes modified by TEOS/O3 chemical vapor deposition
Pore structure evaluation of carbon nanotube and inorganic membranes through sorption and permeability studies
Development of nanostructured carbon and silicon composite membranes for separation of gas mixtures and study of carbon nanostructures for hydrogen storage
In the present Ph.D. thesis, several types of composite nanofiltration (NF) membranes were examined for their ultra thin separation layer integrity by initially applying gas permeability measurements. The examined materials were sol-gel derived α-alumina and silica membranes, supported on macroporous α-alumina tubes. The existence of defects was also confirmed by application of a recently developed mercury intrusion technique, which additionally enabled the accurate definition of the defects size. Furthermore segments of the defect bearing membranes were modified by chemical vapor deposition of SiO2 under several reaction conditions, to examine the possibility of simultaneously reducing the NF pores size and mending the defects. SEM micrographs of the CVD treated membranes revealed that differential pressure across the reactor sides leads to the formation of an extra silica deposit on either the thin layer or the support surface of the membrane. This was especially the case in the presence of large pores (defects) and was further verified by the novel mercury intrusion method. Monitoring and controlling structure evolution of nanoporous materials during their synthesis or modification can be achieved by innovative combinations of in-situ and ex-situ techniques applied in consecutive or real-time modes. An in-situ technique was applied to monitor the gradual alteration of pore size during the CVD treatment of mesoporous (4 nm) membranes (Vycor, γ-Alumina) for which Knudsen diffusion is initially the predominant transport mechanism. The in-situ experimental results (Oxygen permeance) are interpreted in combination with ex-situ experimental results (Nitrogen isotherms at 77 K) acquired after the completion of CVD, to give an insight of the effect of different CVD conditions on pore size alteration. The CVD treatments were performed via an in-house developed CVD reactor that additionally permitted the in-situ monitoring of Oxygen permeance during each CVD process and moreover the application of gas pressure pulses up to 3 bars and the measurement of several single gas permeances (298-650 K) after the deposition stage, without the need of dismantling the membrane tubes. In the present study, tailoring of the pore size of silica membranes was achieved by applying combinations of in-situ gas permeability with mercury porosimetry, SEM and EDAX, during the membrane modification using two CVD modes: in the normal cyclic mode, a sequential exposure to silica precursor vapors and O3 was performed whereas in the pressure-pulsed mode, the exposure to silica precursor was accompanied by pressure pulses of the order of 400 kPa. The membranes were treated with tetramethyl orthosilicate (TMOS) in both modes under different CVD durations. The aim was to examine effects in the pore narrowing rate, deposition profiles and membrane performance characteristics that are induced due to the different physicochemical properties of the involved precursors, as well as due to the different CVD conditions. ......................Στην παρούσα διδακτορική διατριβή, δύο τύποι σύνθετων μεμβρανών νανοδιήθησης εξετάστηκαν ως προς την αποδοτικότητα του λεπτού στρώματος διαχωρισμού, επιτελώντας μετρήσεις διαπέρασης αερίων. Τα υπό εξέταση υλικά ήταν μεμβράνες γ-αργιλίας (με τυπική διάμετρο πόρων 4 και 10 nm) υποστηριγμένες σε μακροπορώδη υποστρώματα α-αργιλίας, καθώς και σωληνωτού τύπου ύαλοι Vycor. Η ύπαρξη ορισμένου αριθμού ατελειών στις σύνθετες μεμβράνες επιβεβαιώθηκε μέσω εφαρμογής μίας ειδικά τροποποιημένης τεχνικής ποροσιμετρίας υδραργύρου, η οποία επιπλέον κατέστησε δυνατό τον ακριβή προσδιορισμό του εύρους των ατελειών. Επίσης, τμήματα των μεμβρανών α- /γ-αργιλίας που έφεραν ατέλειες υπεβλήθησαν σε τροποποίηση μέσω της διεργασίας CVD (Chemical Vapor Deposition) υπό διαφορετικές πειραματικές συνθήκες, ώστε να εξεταστεί η πιθανότητα τόσο της ελάττωσης της διαμέτρου πόρων όσο και της εξάλειψης των μεγάλου μεγέθους ατελειών του στρώματος διαχωρισμού, λόγω της εναπόθεσης SiO2 εντός της πορώδους δομής. Οι μικρογραφίες SEM (Scanning Electron Microscopy) των τροποποιημένων μεμβρανών φανέρωσαν ότι εφαρμογή διαφοράς πίεσης μεταξύ των δύο πλευρών τους οδηγεί στην ανάπτυξη επιπρόσθετου φιλμ εναπόθεσης πυριτίας είτε στην επιφάνεια του λεπτού στρώματος νανοδιήθησης είτε στην επιφάνεια του υποστρώματος. Αυτή η μετατόπιση της ζώνης εναπόθεσης συνέβη ιδιαίτερα στην περίπτωση της παρουσίας μακροπόρων ή ατελειών στο λεπτό στρώμα νανοδιήθησης, γεγονός που επιβεβαιώθηκε από πειράματα της ειδικά αναπτυγμένης τεχνικής ποροσιμετρίας υδραργύρου. Η συνεχής παρακολούθηση και ο έλεγχος της εξέλιξης της πορώδους δομής των μεμβρανών νανοδιήθησης κατά τη διάρκεια της τροποποίησής τους είναι δυνατόν να επιτευχθεί με τη βοήθεια in-situ και ex-situ τεχνικών, που συνδυάζονται είτε σε διαδοχικά στάδια είτε σε πραγματικό χρόνο. Μία τεχνική in-situ αξιοποιήθηκε με σκοπό την παρακολούθηση σταδιακών αλλαγών της διαμέτρου των πόρων κατά την τροποποίηση με βάση τη διεργασία CVD των μεσοπορωδών μεμβρανών α- /γ-αργιλίας και Vycor, για τις οποίες ο μηχανισμός διάχυσης τύπου Knudsen είναι ο αρχικός μηχανισμός μεταφοράς. Τα αποτελέσματα της in-situ καταγραφής της διαπέρασης Ο2 αναλύονται σε συνδυασμό με δεδομένα από ex-situ στατικές πειραματικές τεχνικές (ποροσιμετρία Hg και ποροσιμετρία Ν2 σε 77 Κ), τα οποία ελήφθησαν μετά την ολοκλήρωση κάθε διεργασίας CVD, προκειμένου να διαλευκανθεί η επίδραση των διαφορετικών συνθηκών κατεργασίας στη μεταβολή του εύρους των πόρων. Οι διεργασίες CVD διεξήχθησαν σε μία συσκευή που σχεδιάστηκε και κατασκευάστηκε στο εργαστήριο και η οποία επίσης επιτρέπει τη συνεχή παρακολούθηση της διαπέρασης Ο2 και την εφαρμογή πιέσεων μέχρι 0.3 MPa, καθώς και μετρήσεις διαπέρασης άλλων αερίων. ..........................
Innovative methods for the characterization of ceramic nanofiltration membranes modified by TEOS/O3 chemical vapor deposition
Innovative methods for the characterization of ceramic nanofiltration membranes modified by TEOS/O3 chemical vapor deposition
Pore Structure and Gas Diffusion Features of Ionic Liquid-Derived Carbon Membranes
In the present study, the concept of Ionic Liquid (IL)-mediated formation of carbon was applied to derive composite membranes bearing a nanoporous carbon phase within their separation layer. Thermolytic carbonization of the supported ionic liquid membranes, prepared by infiltration of the IL 1-methyl-3-butylimidazolium tricyanomethanide into the porous network of Vycor® porous glass tubes, was applied to derive the precursor Carbon/Vycor® composites. All precursors underwent a second cycle of IL infiltration/pyrolysis with the target to finetune the pore structural characteristics of the carbonaceous matter nesting inside the separation layer. The pore structural assets and evolution of the gas permeation properties and separation efficiency of the as-derived composite membranes were investigated with reference to the duration of the second infiltration step. The transport mechanisms of the permeating gases were elucidated and correlated to the structural characteristics of the supported carbon phase and the analysis of LN2 adsorption isotherms. Regarding the gas separation efficiency of the fabricated Carbon/Vycor® composite membranes, He/CO2 ideal selectivity values as high as 4.31 at 1 bar and 25 °C and 4.64 at 0.3 bar and 90 °C were achieved. In addition, the CO2/N2 ideal selectivity becomes slightly improved for longer second-impregnation times
Pore structure evaluation of carbon nanotube and inorganic membranes through sorption and permeability studies
Pore structure evaluation of carbon nanotube and inorganic membranes through sorption and permeability studies
Effect of macroscale formation of intraluminal thrombus on blood flow in abdominal aortic aneurysms
A mathematical approach of blood flow within an abdominal aortic aneurysm (AAA) with intraluminal thrombus (ILT) is presented. The macroscale formation of ILT is modeled as a growing porous medium with variable porosity and permeability according to values proposed in the literature. The model outlines the effect of a porous ILT on blood flow in AAAs. The numerical solution is obtained by employing a structured computational mesh of an idealized fusiform AAA geometry and applying the Galerkin weighted residual method in generalized curvilinear coordinates. Results on velocity and pressure fields of independent cases with and without ILT are presented and discussed. The vortices that develop within the aneurysmal cavity are studied and visualized as ILT becomes more condensed. From a mechanistic point of view, the reduction of bulge pressure, as ILT is thickening, supports the observation that ILT could protect the AAA from a possible rupture. The model also predicts a relocation of the maximum pressure region toward the zone proximal to the neck of the aneurysm. However, other mechanisms, such as the gradual wall weakening that usually accompany AAA and ILT formation, which are not included in this study, may offset this effect. © 2014 Taylor & Franci